Polyadenylic acid (poly(A)) was incorporated into polyacrylamide gels and utilized as a substrate for human serum ribonucleases after electrophoresis. This was accomplished by selecting conditions for electrophoresis such that the enzymes had no activity. After electrophoresis, gels were incubated in buffer to induce hydrolysis of poly(A) in the narrow regions to which the RNases migrated. Staining revealed these regions of enzyme activity. Eight bands of activity were consistently found in the sera of 19 volunteer blood donors. All samples from this group gave a band of activity at Rf equals 0.39. This enzyme activity was totally absent in the sera of 13 patients having kidney disease of varied etiology (CCR 5 ml/min.). However, all of these patients had no distinct activity at Rf equals 0.33, not found in sera from normal individuals. Although the activity patterns measured using sera of patients having multiple myeloma differ from the normal pattern, the activity associated with decreased renal function was found in 3/3 patients having renal insufficiency concomitant with myeloma. Other patients (8/9) with higher CCR values, had only the activity in this region found in normal volunteers. Currently, an activity band useful in the detection or grading of various types of tumor is being sought.